The invention relates to an electrical isolation device capable of coupling together two circuits at frequencies from direct current (DC) up to high frequencies, which two circuits are at different electrical potentials, and the invention notably applies to electrical measuring instruments such as voltmeters and oscilloscopes.
A problem arises when the source of a signal to be measured and the measuring instrument itself do not share a common ground reference.
Floating-ground operation of the measuring instrument is not a satisfactory solution, firstly because the accuracy of the measurement may be affected by the presence of ground currents, and secondly because there is a risk of the potential of the measuring instrument reaching a dangerous level.
Proposals have therefore been made to transmit a signal that is to be measured across an electrical isolation barrier disposed between the source of the signal and the measuring instrument, which source and instrument may then retain their respective ground references, with floating-ground operation of the measuring instrument thus being avoided.
An analog isolation device incorporating such an electrical isolation barrier and shown diagrammatically in FIG. 1 is described in the document EP 0 875 765.
That known device 10 comprises a primary part 12 and a secondary part 14 isolated from each other by an isolation barrier 16. As shown in FIG. 1, the primary and secondary parts are connected to respective independent grounds.
The device 10 receives an input signal at an input A of the primary part 12 and is designed to deliver an isolated output signal at an output B of the secondary part 14, reproducing the input signal 16. To this end, the device 10 must have a flat frequency response, that is to say it must supply an output signal of amplitude that is identical (ignoring any multiplier coefficient) to the amplitude of the input signal, and it must do so across the whole of the range of usable frequencies.
To this end, the device comprises two parallel channels, a high-frequency (HF) channel conveying the HF component of the input signal and a low-frequency (LF) channel conveying the LF component of the input signal, the output signal being obtained by summing the LF and HF components that reach the secondary part.
The HF channel includes a transformer 18 having its primary P connected to the input A. On its secondary winding S, the transformer 18 faithfully reproduces the HF component of the input signal, but its frequency response deteriorates rapidly at low frequencies.
The LF channel includes an opto-coupler 20 comprising an electro-optical emitter 20a, such as a light emitting diode (LED), connected to the input A and coupled on the secondary side to an opto-electric receiver, such as a photodiode 20b, that delivers current that is converted to a voltage by a circuit 22. The opto-coupler 20 further includes a second opto-electric receiver, such as a photodiode 20c similar to the photodiode 20b and also coupled to the electro-optical emitter 20a, but situated on the primary side, the current from the photodiode 20c being converted into a voltage by a circuit 24. The output voltage of the circuit 24 is applied to the inverting input of an amplifier 26 receiving the input signal on its non-inverting input, the circuit 24 and the amplifier 26 forming a feedback loop for linearizing the response of the opto-coupler 20. The opto-coupler 20 faithfully reproduces the LF component of the input signal, but its frequency response deteriorates rapidly at high frequencies.
The transformer 18 and the opto-coupler 20 form the electrical isolation barrier 16. The output signal is obtained by summing the HF component on the secondary of the transformer 18 and the LF component at the output of the circuit 22 by means of a circuit 28.
FIG. 2 is a Bode diagram showing the frequency responses H1(f) and H2(f) of the LF and HF channels, respectively.
To obtain a flat overall frequency response, that is to say for the output signal faithfully to reproduce the input signal throughout the range of useful frequencies, it is necessary for there to be a corresponding relationship between the high cut-off frequency FLF1 of the LF channel (opto-coupler 20) and the low cut-off frequency FHF1 of the HF channel (transformer 18).
To this end, a fraction of the output voltage of the circuit 24 as determined by a divider circuit 30 is applied to the non-inverting input of an amplifier 32 having its inverting input receiving the input signal and having its output connected to primary P of the transformer 18. The output voltage of the circuit 24 is an image of the LF component transmitted to the secondary via the opto-coupler 20. The division ratio of the circuit 30 is adjusted to subtract from the input signal a fraction of the LF component such that the cut-off frequency FHF is aligned with the cut-off frequency FLF.
The Applicant has nevertheless determined that that technique of compensating misalignment of the cut-off frequencies of the LF and HF channels does not guarantee a totally satisfactory result, that is to say the absence of any significant distortion of the output signal compared to the input signal. It is difficult to adjust the division ratio of the circuit 30 in an optimal fashion. Moreover, there is no compensation of asymptotic response errors in the LF channel resulting from the presence of orders higher than 1 in the cut-off frequency of that channel. Moreover, since the compensation is effected by aligning the cut-off frequency of the HF channel with that of the LF channel, the LF channel operates at full bandwidth, and it is necessary for the output from the opto-coupler going respectively to the secondary part via the photodiode 20b and to the primary part via the photodiode 20c to have the same bandwidth and the same gain, which requires delicate adjustments and components to be chosen that have small differences between their characteristics.